The ingress of moisture into the interstices of an electric cable consisting of a multiplicity of plastic insulated conductors poses a serious problem. Depending on the type of core involved and the construction of the cable it disrupts transmission characteristics to different degrees and can result in some cases in complete failure of the cable. There are a number of mechanisms by which water can enter such a cable, sheath and joint failure being but two examples.
Once inside the sheath water can create a variety of effects. In general water entering a pulp and/or paper insulated cable will result in short circuits occurring between the conductors. However, the phenomenon of swelling of the paper or pulp tends to prevent further penetration along the core, thus localising the fault. In air core plastic insulated cable the water is free to travel by capillary action along the cable interstices and can result in large capacitance increases.